A Google search will turn up plenty of stenciling instructions and videos. Here are the basics for roller stenciling with a single color, from Cutting Edge Stencils (www.cuttingedgestencils.com):
• Adhere the stencil to the surface with blue painter’s tape or spray adhesive.
• Pour a few tablespoons of paint onto a foam plate or paint tray. Load a foam roller with paint, and then roll off most of the paint on a paper towel or other absorbent paper until the roller is almost dry.
• Roll the paint over the stencil. Don’t press too hard, or you might force paint under the stencil’s edges.
• If the design is too light, let the paint dry a few minutes and repeat.
• Remove the stencil carefully.
• If you’re using the stencil repeatedly, clean the stencil when paint starts to build up and compromise your design.